The present invention relates to a shut off plate assembly for use in a sliding closure unit or a slide gate mounted on a vessel containing molten metal for controlling the discharge of the molten metal from such vessel. Particularly, the present invention relates to such a shut off plate assembly of the type including a refractory plate having therethrough a flow or discharge opening, and an insert surrounding the discharge opening and to enable a gas to be injected into the discharge opening.
German DE-PS 19 38 117 discloses a slide gate including a stationary shut off plate assembly of the above type and including an insert in the form of a gas permeable annular sleeve surrounding the discharge or flow opening. Thus, when the slide gate is closed, gas can be injected through the gas permeable annular sleeve into the discharge opening, thereby preventing molten steel from solidifying within a discharge spout of the metallurgical vessel. Additionally, gas can be injected through the gas permeable annular sleeve when the slide gate is in an opened position, for example during a pouring or discharge operation, for the purpose of preventing molten steel from clogging the discharge opening and also for the purpose of forming a seal to prevent the molten steel that is being discharged from absorbing exterior oxygen or air.
However, in this known arrangement, the gas permeable annular sleeve is formed of a relatively porous refractory material and as such has a relatively low resistance to abrasion and erosion that occurs during molten metal discharge. The refractory plate on the other hand normally is formed of a highly refractory material, for example usually compressed or more dense, that has a greater resistance to abrasion and erosion. As a result, material of the plate assembly in the region of the annular sleeve very rapidly can be washed out or eroded away. As a consequence, the entire plate assembly becomes useless and must be taken out of service. This particularly is disadvantageous when the plate assembly is a bottom or stationary plate assembly.